Virtually all systems that manipulate analog, digital or mixed signals utilize at least one reference voltage as a basis for other operations in the system. The reference voltage should be reproducible every time the circuit is powered up, and must remain relatively unchanged over process, voltage and temperature (PVT) variations. A well known conventional technique for producing a reference voltage is the semiconductor bandgap reference circuit, also referred to as a bandgap reference. A bandgap reference circuit relies on the bandgap energy of the underlying semiconductor material. Such bandgap reference circuits are well known in the art. Conventional examples of such bandgap reference circuits can be found, for example, in U.S. Pat. Nos. 6,075,407 and 6,281,743, both of which are incorporated herein by reference.
In addition to the aforementioned requirement that bandgap reference circuits provide adequate performance over PVT variations, many applications also require the bandgap reference circuit to operate at vary low supply currents, and to be as small as possible.
One factor that can significantly impact the performance of a bandgap reference circuit is the input offset of the control loop amplifier (error amplifier). The offset is typically multiplied by approximately 10 in most bandgap designs. This multiplication factor can translate a relatively small offset error at the amplifier input into a relatively large error at the amplifier output. Trimming can provide a limited solution to the offset problem, but usually only at a single nominal temperature, not across the entire operating temperature range. Choppers have is also been conventionally utilized to solve offset problems, but they are not particularly effective in some applications, for example, low drop out amplifiers (LDOs) and switched power supplies.
It is desirable in view of the foregoing to provide bandgap reference circuit designs which provide adequate performance over PVT variations, operate at low currents, produce a relatively compact circuit footprint, and overcome offset problems.